Bacteroides fragilis is the leading cause of anaerobic bacteremia and intraabdominal abscesses. Studies have shown that the interaction of the B. fragilis capsular polysaccharide complex (CPC) with the host immune system leads to the formation of intraabdominal abscesses. The B. fragilis CPC is composed of at least five distinct capsular polysaccharides, polysaccharide A, B, C, D, and E (PS A-E). Each of these capsular polysaccharides is able to undergo a reversible ON/OFF phenotype known as phase variation. The biosynthesis loci containing the genes for the synthesis of PS A, PS B and PS C have been completely sequenced as well the region upstream of the PS D locus. These sequences have allowed for the analysis of the mechanism phase variation of these capsular polysaccharides at the genetic level. Preliminary data suggest that variation of the B. fragilis capsular polysaccharides occurs due to the DNA inversion of a small segment of DNA upstream of the respective polysaccharide biosynthesis locus. The invertible regions upstream of the PS A and PS D loci have been demonstrated to contain functional promoters. This application is divided into aims that will analyze phase variation of these capsular polysaccharides at the DNA, RNA and polysaccharide levels. Studies will be conducted to characterize the cis-acting elements necessary for DNA inversion of promoter regions and the trans-acting factors that mediate inversion. Additionally, the characteristics of phase variation of each of these capsular polysaccharides will be analyzed from organisms grown in vitro, and isolated from in vivo sites associated with health (colon) and disease (peritoneal cavity and abscesses) Due to the novelty of the phase variation of five distinct capsular polysaccharide by a single organism, we have the unique opportunity to determine if there is coordinate regulation between the expression of distinct capsular polysaccharides, and the role phase variation of the B. fragilis capsular polysaccharides may play in health and disease.